- With the rack in the middle position, preheat the oven to 200 °C (400 °F).
- In a food processor, combine flour, sugar and baking powder. Add butter and pulse until the mixture has the texture of sand. Add egg and pulse until dough has a grainy texture. With your fingertips, press dough at the bottom and edges of a 23-cm (9-inch) pie plate. Bake for about 22 minutes or until golden brown. Let cool.
- In a saucepan, off the heat, combine sugar and cornstarch. Stir in egg yolks, lemon zest and juice, stirring with a whisk. Add water and bring to a boil, stirring constantly. Simmer for one minute and remove from heat. Strain. Add butter and stir until melted.
- Pour into crust. Cover with plastic wrap directly on filling and let cool in the refrigerator.
- In a large bowl, over a double boiler, off the heat, combine water and cornstarch. Cook, stirring with a whisk until the mixture thickens. Remove the bowl from the double boiler. Stir in sugar and egg whites. Put the bowl back on the double boiler and heat for about 1 minute or until sugar has dissolved. Remove the bowl from the double boiler and beat with an electric mixer until meringue forms semi-firm peaks. Cover pie with meringue and bake in the centre of the oven at 180 °C (350 °F) for about 15 minutes or until meringue is golden brown. Refrigerate for 6 to 8 hours.